1. Field of the Invention
The present invention relates to automatic focusing mechanisms for operation microscopes and, particularly, to an automatic focusing mechanism for an operation microscope, having an origin returning function.
2. Description of the Prior Art
Local operation of an affected or diseased part in a cerebral surgical operation and a heart surgical operation is executed while observing the affected part by an operation microscope, and is generally called "microsurgery". FIG. 2 of the attached drawings shows an example of the use of such conventional operation microscope (refer to Japanese Patent Laid-Open No. SHO 64-56409, id est, U.S. Pat. No. 4,881,709).
The reference numeral 1 denotes an operation microscope. The operation microscope 1 is mounted on a depending arm 2 of a stand apparatus (not shown), whereby the operation microscope 1 is movable in any direction including front and rear directions, upper and lower directions and left- and right-hand directions, and an observation angle .theta. can also be altered freely.
That is, a switch section 5, which is provided on a grip section 4 of an operating arm 3, is depressed whereby the operation microscope 1 is brought to a free condition as if the operation microscope 1 floats in the air. Accordingly, an operator D sets the operation microscope 1 to a desired location or position while the switch section 5 is depressed. Then, when the operator D leaves or detaches his hand from the switch section 5, an electromagnetic clutch (not shown) operates so that the operation microscope 1 is brought to such a condition as to be locked at its position.
When the operation microscope 1 is fixed to a predetermined position, an objective lens (optical element) 7 moves along a focus control direction F by an automatic focusing mechanism 6 which is incorporated in the operation microscope 1, so that the focus is taken to an observation target point X on a patient P.
FIG. 3 shows a structure or construction of the automatic focusing mechanism 6. The reference numeral 8 denotes a support section by which control of the objective lens 7 is executed. That is, within the support section 8, the automatic focusing mechanism 6 is provided which comprises upper- and lower-side limit switches 9 and 10, a drive objective section 11 in interlocking relation to the objective lens 7, and drive means 12 for reciprocating the drive objective section 11 within a moving extent or range E between the upper-side limit switch 9 and the lower-side limit switch 10. The drive means 12 moves the drive objective section 11 along the focus control direction F, whereby the objective lens 7 is also moved in interlocking relation thereto, thereby executing zoom regulation or adjustment.
That is, a foot switch 12a, which is located at feet of the operator D, is connected to the drive means 12. After the operation microscope 1 has been brought to the lock condition (a condition under which a hand is released or detached from the switch section 5), the foot switch 12a is depressed whereby the drive objective section 11 is moved toward the upper limit switch 9 or toward the lower limit switch 10 so that zoom adjustment of a field of view can be executed.
In the case of such conventional automatic focusing mechanism for the operation microscope, however, a problem illustrated in FIG. 4 arises. That is, after the switch section 5 has been depressed to bring the operation microscope 1 to a free condition, when the operation microscope 1 is moved to a point different from the observation target point X which has been seen until now, and when the operation microscope 1 is brought to a locked condition at the point to see or look the point, a problem illustrated in FIG. 4 arises.
That is, the operator D depresses the switch section 5 during operation to alter a position of the operation microscope 1 several times, and operates the foot switch 12a every position to execute zoom adjustment. For this reason, the operator D does not accurately grasp at what position within the moving range E the drive objective section 11 is located. Accordingly, as shown in FIG. 4, it is desired for the operator D to operate the foot switch 12a thereby moving the objective lens 7 (drive objective section 11) downwardly correspondingly to EX. Since, however, a downwardly moving margin E1 of the drive objective section 11 has only a small amount of room left, there may occur a situation that desired zoom adjustment cannot be executed.